The present invention relates generally to the blow molding of thermoplastic material and, more particularly, to a method of blow molding plastic articles in combination with a transfer molding technique. While not limited thereto, this invention is particularly concerned with the molding of plastic articles of one-piece construction having at least one wall section of a different thickness than the wall thickness of the major portion of the finished article.
It has been found that blow molding is a relatively inexpensive and efficient process for molding hollow plastic articles or articles of a double wall construction having substantially uniform wall thicknesses. However, a problem arises where it is desired to vary the wall thickness of certain portions of the article being formed, especially where a wall section of a thickness greater than the wall thickness of the parison is desired. Also, the flowing thermoplastic material in a blow molding operation tends to thin out as the material is stretched during blowing, this thinning out being especially pronounced at relatively sharp corners producing weaker portions at and adjacent to such corners. Moreover, voids or spaces are formed along the mold surfaces adjacent these corners to produce irregularities in the finished surfaces. This is especially undesirable where such surfaces are intended as sealing surfaces.